In planetary systems with two or more giant planets , dynamical instabilities can lead to collisions or ejections through strong planet–planet scattering .
Previous studies for simple initial configurations with two equal-mass planets revealed two discrepancies between the results of numerical simulations and the observed orbital elements of extrasolar planets : the potential for frequent collisions between giant planets and a narrow distribution of final eccentricities following ejections .
Here , we show that simulations with two unequal mass planets starting on nearly circular orbits predict a reduced frequency of collisions and a broader range of final eccentricities .
We show that the two-planet scattering model can easily reproduce the observed eccentricities with a plausible distribution of planet mass ratios .
Further , the two-planet scattering model predicts a maximum eccentricity of \simeq 0.8 , independent of the distribution of planet mass ratios , provided that both planets are initially place on nearly circular orbits .
This compares favorably with current observations and will be tested by future planet discoveries .
Moreover , we show that the combination of planet–planet scattering and tidal circularization may be able to explain the existence of some giant planets with very short period orbits .
Orbital migration due to planet scattering could play an important role in explaining the increased rate of giant planets with orbital periods of less than a year , as found by radial velocity surveys .
We also re-examine and discuss various possible correlations between eccentricities and other properties of observed extrasolar planets .
We find that the radial velocity observations are consistent with planet eccentricities being correlated with the ratio of the escape velocity from the planet ’ s surface relative to the escape velocity from the host star at the planet ’ s location .
We demonstrate that the observed distribution of planet masses , orbital periods , and eccentricities can provide constraints for models of planet formation and evolution .